Bicycle derailleurs are used to effect gear shifts by changing the position of a drive chain between the variable diameter sprockets of a multi-sprocket freewheel or crankset. The position of a chain on a sprocket of a freewheel and on a chainring of a crankset determines the gear ratio of the bicycle at which the rider must pedal. Typically, as shown in FIG. 1, a rear derailleur 10 includes a parallelogram linkage 20 that is attached to a bicycle frame element 30 at one end, and to an idler cage 32 at another end, the idler cage laterally urging the chain between the multiple sprockets or chainrings (not shown). A control cable 34 is tied at one end to a shifter mounted proximate the rider's hand on the handlebar or on the downtube of a bicycle frame (not shown), and at the second end to the parallelogram. The parallelogram is typically spring biased in a given direction, requiring the rider to actuate the shifter and tension the control cable in order to deflect the parallelogram and in turn urge the chain in a direction opposite to the spring- biased direction.
A parallelogram linkage 20 typically includes inner and outer links or side plates 40, 42, respectively, pivotally connected at axes D and C, respectively, to a b-knuckle 44 (which in turn is attached to the bicycle frame 30) and pivotally connected at axes B and A, respectively, to a p-knuckle 46 (to which the idler cage 32 is attached), the parallelogram displaceable to precisely position the idler cage 32 and in turn the drive chain onto the desired sprocket. A return spring 48 biasing the parallelogram is typically a helical or coil spring urging together diagonally opposed pivot points of the parallelogram.
As shown in FIG. 2, links are typically formed into longitudinal channel members from a flat plate with pivot holes 50-53 located at both ends of resulting flange elements 54 for pivotal attachment to b-knuckles 44 and p-knuckle 46 with a pair of pins (not shown). In such link designs, it is difficult to control the angle of bend of the flange elements 54 with respect to a base element 55 and in turn the overall width W of the link member 40 (FIG. 2a). Accordingly, this may result in binding between the link 40 and knuckle elements 44, 46, inhibiting the smooth operation of the parallelogram and potentially even jamming the derailleur. Additionally, as shown in FIGS. 2b (top view) and 2c (end view), the four pivot holes 50-53 punched into the flanges 54 must be precisely located to ensure proper alignment of the holes both along the length and height of the flanges to ensure "parallelism" of the pivot holes. Again, failure to hold close tolerances in locating the holes will make assembly of links 40, 42 onto the knuckles 44, 46 difficult and inhibit the smooth displacement of the parallelogram 20.
A further consideration in link designs is described in this assignee's pending application Ser. No. 09/005,214, entitled "Hybrid Spring For Bicycle Derailleurs." In one embodiment of the invention of said application, a hybrid derailleur return spring force is achieved by urging an abutment member attached to one of the links forming the parallelogram against the coil spring during actuation of the parallelogram thereby imparting a transverse force to the coil spring and laterally deflecting the coil spring to produce the desired spring force. In such designs, the extent of lateral displacement of the spring is restricted by contact with the base element 55 of conventional link configurations thereby limiting the spring forces that can be achieved.
A need therefore exists for a derailleur parallelogram link that is sufficiently rigid axially to maintain the distance between the pivot axes during derailleur actuation but is also sufficiently flexible in torsion and shear to permit simple alignment of the pivot holes and easy control of the link width. A need also exists for parallelogram links that permit extended lateral displacement of coil return springs for parallelograms configured with abutment members that laterally deflect the return spring.